More Accurate Type Checking Than the typeof Operator

The return values of Object.prototype.toString for different data types are as follows:

• Number: returns [object Number].
• String: returns [object String].
• Boolean: returns [object Boolean].
• undefined: returns [object Undefined].
• null: returns [object Null].
• Array: returns [object Array].
• arguments object: returns [object Arguments].
• Function: returns [object Function].
• Error object: returns [object Error].
• Date object: returns [object Date].
• RegExp object: returns [object RegExp].
• Other objects: returns [object Object].

This means that Object.prototype.toString can reveal the actual type of a value.

Object.prototype.toString.call(2) // "[object Number]"
Object.prototype.toString.call('') // "[object String]"
Object.prototype.toString.call(true) // "[object Boolean]"
Object.prototype.toString.call(undefined) // "[object Undefined]"
Object.prototype.toString.call(null) // "[object Null]"
Object.prototype.toString.call(Math) // "[object Math]"
Object.prototype.toString.call({}) // "[object Object]"
Object.prototype.toString.call([]) // "[object Array]"

Using this characteristic, you can write a type-checking function that is more accurate than the typeof operator.

var type = function (o){
    var s = Object.prototype.toString.call(o)
    return s.match(/\[object (.*?)\]/)[1].toLowerCase()
}
type({}); // "object"
type([]); // "array"
type(5); // "number"
type(null); // "null"
type(); // "undefined"
type(/abcd/); // "regex"
type(new Date()); // "date"

Building on this type function, you can also add methods that check for specific data types.

var type = function (o){
  var s = Object.prototype.toString.call(o);
  return s.match(/\[object (.*?)\]/)[1].toLowerCase();
};

['Null',
 'Undefined',
 'Object',
 'Array',
 'String',
 'Number',
 'Boolean',
 'Function',
 'RegExp'
].forEach(function (t) {
  type['is' + t] = function (o) {
    return type(o) === t.toLowerCase();
  };
});

type.isObject({}) // true
type.isNumber(NaN) // true
type.isRegExp(/abc/) // true